1. Field of the Invention
The present invention relates to a liquid crystal display, and more particularly, to a transmissive and reflective type liquid crystal display in which the display operation is carried out in reflection mode and transmission mode.
2. Description of the Related Art
Liquid crystal displays (LCDs) have become displays of choice among the various developed flat panel type displays because they are much slimmer and lighter than other types of displays. They also require lower driving voltage and lower power consumption.
LCD displays are classified as transmission type which display images using an external light source such as a backlight, as reflection type which display an image using natural light, and transmissive and reflective type which display in a transmission mode using an internal light source provided in the display itself at indoors or a dark place where an external light source does not exist and the display operates in a reflection mode to display images by reflecting external incident light in a high brightness environment such as at outdoors.
LCDs can also be classified depending on the way they are driven. For example, In the passive matrix type, pixels in the LCDs are driven using a root-mean-square (rms) of a difference between voltages applied to signal lines and scanning lines, while a line addressing in which a signal voltage is applied to all of the pixels at the same time is carried out. In the active matrix type, pixels are driven by a switching element such as a MIM (Metal-insulator-metal) device or a thin film transistor.
FIG. 1 is a sectional view of a conventional transmissive and reflective type LCD, and shows an active matrix type LCD using the thin film transistor.
Referring to FIG. 1, the conventional transmissive and reflective type LCD includes a first substrate 10, a second substrate 40 arranged facing the first substrate 10, a liquid crystal layer 50 formed between the first substrate 10 and the second substrate 40, and a light source, i.e., a backlight assembly 60 disposed at a rear side of the first substrate 10.
The first substrate 10 includes a first insulating substrate 11, a thin film transistor 25 formed on the first insulating substrate 11, a passivation film 30 having a contact hole 32 for exposing a part of the thin film transistor 25, a transparent electrode 34, and a reflection electrode 36. The thin film transistor 25 includes a gate electrode 12, a gate insulating film 14, an active pattern 16, an ohmic contact pattern 18, a source electrode 20, and a drain electrode 22. The transparent electrode 34 functions as a pixel electrode for transmitting light that is generated from the backlight 60 and is then incident through the first substrate 10. The transparent electrode 34 is connected to the thin film transistor 25 formed on every unit pixel region on the first substrate 10. The reflection electrode 36 reflects external light that is incident through the second substrate 40 and at the same time functions as another pixel electrode. The transparent electrodes 34 include regions of a transmission part T and a reflection part R for reflecting the external light incident through the second substrate 40.
The second substrate 40 includes a second insulating substrate 42, a color filter 44 comprised of RGB pixels for displaying colors while light is transmitted therethrough, a black matrix 46 for preventing the light from being leaked between the pixels, and a transparent common electrode 48.
The liquid crystal layer 50 is made of 90xc2x0 twisted nematic (TN) liquid crystal, and has an approximately 0.24 of xcex94nd which is a product of anisotropy xcex94n in refractive index and thickness d of the liquid crystal layer 50.
Also, according to an alignment direction of the liquid crystal molecules, a first polarizing plate 54 and a second polarizing plate 58 are respectively attached to external surfaces of the first and second substrates 10 and 40 so as to transmit only polarized light in a specific direction. The first and second polarizing plates 54 and 58 are all linear polarizers in which each polarizing axis of the first and second polarizing plates 54 and 58 is orthogonal to each other.
Between the first substrate 10 and the first polarizing plate 54, and between the second substrate 40 and the second polarizing plate 58, there are respectively arranged a first xc2xc wavelength phase difference plate 52 and a second xc2xc wavelength phase difference plate 56. Each of the xc2xc wavelength phase difference plates 52 and 56 functions to convert linearly polarized light to circularly polarized light; or vice versa by causing a phase difference of xc2xc wavelength between two polarization components that are orthogonal to each other and are parallel to the optical axes of the xc2xc wavelength phase difference plates 52 and 56.
Hereinafter, there are respectively described operations in the reflection mode and the transmission mode in the conventional transmissive and reflective type LCD shown in FIG. 1.
FIGS. 2A and 2B are schematic views for illustrating an operation of the conventional LCD in the reflection mode.
First, when a pixel voltage is not applied (OFF), as shown in FIG. 2A, light that is incident from an outside is transmitted through the second polarizing plate 58, so that the light is linearly polarized in a direction parallel to the polarizing axis of the second polarizing plate 58. The linearly polarized light is transmitted through the second xc2xc wavelength phase difference plate 56, so that the linearly polarized light is converted onto left-handed circularly polarized light. The left-handed circularly polarized light is transmitted through the liquid crystal layer 50, so that the left-handed circularly polarized light is linearly polarized in a direction vertical to the polarizing axis of the second polarizing plate 58, and is then incident onto the reflection electrode 36. The linearly polarized light, which is reflected by the reflection electrode 36, is transmitted through the liquid crystal layer 50, so that the linearly polarized light is converted onto the left-handed circularly polarized light. The left-handed circularly polarized light is transmitted through the second xc2xc wavelength phase difference plate 56, so that the left-handed circularly polarized light is linearly polarized in a direction parallel to the polarizing axis of the second polarizing plate 58. And then, the linearly polarized light is transmitted through the second polarizing plate 58, so that a white image is displayed.
When a maximum pixel voltage is applied (ON), as shown in FIG. 2B, light that is incident externally is transmitted through the second polarizing plate 58, so that it is linearly polarized in a direction parallel to the polarizing axis of the second polarizing plate 58. The linearly polarized light is transmitted through the second xc2xc wavelength phase difference plate 56, so that it is converted onto left-handed circularly polarized light. The left-handed circularly polarized light is transmitted through the liquid crystal layer 50 without variation in the polarization state, and is then incident onto the reflection electrode 36. The light, which is incident onto the reflection electrode 36, is reflected by the reflection electrode 36, so that it is converted to right-handed circularly polarized light and the converted right-handed circularly polarized light is transmitted through the liquid crystal layer 50. Thus, the right-handed circularly polarized light, which has been passed through the liquid crystal layer 50, is transmitted through the second xc2xc wavelength phase difference plate 56, so that it is linearly polarized in a direction perpendicular to the polarizing axis of the second polarizing plate 58. The linearly polarized light is shielded by the second polarizing plate 58, so that a black image is displayed.
FIGS. 3A and 3B are schematic views for illustrating an operation mechanism of the transmission mode.
When a pixel voltage is not applied (OFF), as shown in FIG. 3A, light that is irradiated from a backlight disposed below the first polarizing plate 54 is incident onto the first polarizing plate 54, and only light propagating in a direction parallel to the polarizing axis of the first polarizing plate 54 is transmitted through the first polarizing plate 54. At this time, since the polarizing axis of the first polarizing plate 54 is perpendicular to that of the second polarizing plate 58, the light that has been passed through the first polarizing plate 54 is converted onto light linearly polarized in a direction perpendicular to the polarizing axis of the second polarizing plate 58. The linearly polarized light is converted onto a right-handed circularly polarized light by a first xc2xc-wavelength phase difference plate 52. The right-handed circularly polarized light is transmitted through a transparent electrode 34, and is then incident to a liquid crystal layer 50. The right-handed circularly polarized light is transmitted through the liquid crystal layer 50, so that it is linearly polarized in a direction parallel to the polarizing axis of the second polarizing plate 58. The linearly polarized light is transmitted through a second xc2xc-wavelength phase difference plate 56, so that it is converted onto the right-handed circularly polarized light. At this time, since only a light component propagating in a direction parallel to the polarizing axis of the second polarizing plate 58 can be transmitted through the second polarizing plate 58, only about 50% of the right-handed circularly polarized light is transmitted through the second polarizing plate 58. Accordingly, there is a light loss of about 50%, and an image having a moderate brightness is displayed.
Meanwhile, although not shown in the drawings, an optical path of the incident light becomes different at a region where a metal layer, such as the gate line, the data line, or the reflection electrode exists in the transmission mode. In other words, light that is incident from the backlight is transmitted through the first polarizing plate 54, so that it is linearly polarized in a direction parallel to the polarizing axis of the first polarizing plate 54. The linearly polarized light is transmitted through the first xc2xc wavelength phase difference plate 52, so that it is right-handed circularly polarized. The right-handed circularly polarized light is reflected by metal layers, and become left-handed circularly polarized. Then, the left-handed circularly polarized light is transmitted through the first xc2xc wavelength phase difference plate 52, so that it is linearly polarized in a direction parallel to the polarizing axis of the first polarizing plate 54. Accordingly, the linearly polarized light is absorbed in the first polarizing plate 54, and does not return to the backlight. Thus, the light reflected by the metal layers is not reproduced and disappears, so that an overall light efficiency is lowered.
When a maximum pixel voltage is applied (ON), as shown in FIG. 3B, light that is irradiated from a backlight disposed below the first polarizing plate 54 is incident onto the first polarizing plate 54, so that only light propagating in a direction parallel to the polarizing axis of the first polarizing plate 54 is transmitted through the first polarizing plate 54. The light linearly polarized by the first polarizing plate 54 is converted into a right-handed circularly polarized light after being transmitted through the first xc2xc wavelength phase difference plate 52. The right-handed circularly polarized light is transmitted through the transparent electrode 34, and is then incident onto the liquid crystal layer 50. The right-handed circularly polarized light is transmitted through the liquid crystal layer 50 without variation in the polarization state, and is linearly polarized in a direction orthogonal to the polarizing axis of the second polarizing plate 58 after being transmitted through the second xc2xc wavelength phase difference plate 56. Afterwards, the light linearly polarized in the direction orthogonal to the polarizing axis of the second polarizing plate 58 is not transmitted to the second polarizing plate 58, so that a dark image is displayed.
As described above, since the conventional transmissive and reflective type LCD has to be provided with the wide band xc2xc wavelength phase difference plates 52 and 56 covering an overall frequency band of the visible ray, as well as the first and second polarizing plates 54 and 58 with respect to each of the first and second substrates 10 and 40, manufacturing cost is increased as compared with the transmission type LCD. Also, since the polarization characteristic in the transmission mode causes light loss of about 50%, there are drawbacks in that a light transmissivity decreases by about 50% and contrast ratio (C/R) is lowered.
Further, since xcex94nd of the liquid crystal layer 50 is only about 0.24 xcexcm which is a half of xcex94nd (about 0.48 xcexcm) of the conventional transmission type LCD, the cell gap of the liquid crystal cell should be decreased to a level of about 3 xcexcm, and the refractive anisotropy xcex94n of the liquid crystal also should be decreased. Accordingly, there is a need for a transmissive and reflective type LCD device and method which avoids aforementioned problems.
Accordingly, the present invention is to solve the aforementioned problems of the conventional art, and it is an object of the present invention to provide a transmissive and reflective type LCD capable of simplifying a structure of a liquid crystal cell and decreasing light loss in the transmission mode.
In one aspect, there is provided a transmissive and reflective type LCD comprising a first substrate, a second substrate, a liquid crystal layer, a first polarizing plate, a second polarizing plate, a backlight, and a transparent transflective film. In the transmissive and reflective type LCD, the second substrate has an inner surface that is arranged facing the first substrate, and the liquid crystal layer is formed between the first substrate and the second substrate. The first polarizing plate is formed on an outer surface of the first substrate. The second polarizing plate is formed on an outer surface of the second substrate, the outer surface being opposite to the inner surface of the second substrate. The backlight is arranged for irradiating incident light onto the first polarizing plate. The transparent transflective film is arranged between the first polarizing plate and the backlight for partially reflecting and partially transmitting the incident light. The transparent transflective film includes at least a first layer and a second layer having different refractivity indexes from each other and are alternatively stacked.
According to another aspect of the invention, there is provided a transmissive and reflective type LCD for partially reflecting and transmitting incident light, comprising an LC cell, a first polarizing plate, a second polarizing plate, a backlight and a transparent transflective film. The LC cell includes a first substrate, a second substrate having an inner surface that is arranged to face the first substrate, and a liquid crystal layer formed between the first substrate and the second substrate. The first polarizing plate is formed on an outer surface of the first substrate. The second polarizing plate is formed on an outer surface of the second substrate that oppositely faces the inner surface of the second substrate. The backlight is arranged at a rear side of the first polarizing plate. The transparent transflective film is arranged between the first polarizing plate and the backlight, and has a plurality of layers in which a first layer and a second layer having different refractivity indexes from each other. The transmissive and reflective type LCD has a reflection light path along which the incident light is incident onto the LC cell from a front side of the LC cell, is reflected by the transflective film, and is output through the front side of the LC cell. And, the transmissive and reflective type LCD has a transmission light path along which the incident light is incident onto the LC cell from a rear side of the LC cell, is transmitted through the transflective film, and is output through the front side of the LC cell.
The transmissive and reflective type LCD of the invention does not require a reflection electrode within LC cell or a xc2xc-wavelength phase difference plate on each of the upper substrate (second substrate) and the lower substrate (first substrate). Hence, compared with the conventional transmissive and reflective type LCD, the transmissive and reflective type LCD of the present invention is simpler and more easily made.
Further, it is possible that the transflective film of partially being transmitted and reflecting incident light performs both functions of the reflection electrode and the transparent electrode at the same time, and a recycling process of light is lastingly generated, so that light loss is not generated in the transmission mode. Accordingly, compared with the conventional transmissive and reflective type LCD, the transmissive and reflective type LCD of the invention has an enhanced transmissivity. Also, since the transmissive and reflective type LCD of the invention does not utilize the xc2xc wavelength phase difference plate, the light that is incident from the backlight and is then reflected by metal regions of LC cell is recycled and again used, so that it becomes possible to enhance an overall light efficiency.
Furthermore, since the optical conditions applied to the liquid crystal of the conventional transmissive and reflective type LCD can be identically applied to that of the transmissive and reflective type LCD of the present invention, there is no degradation in reliability.